Absorption and emission spectrum

It is well known that main group metal ions with an s* electron configuration form a large variety of polynuclear coordination compounds [l]. Recently it has been suggested that in these compounds a weak metal-metal bonding may exist which is achieved by sp orbital mixing [2]. In particular, calculations on Tl+ clusters were used to support this idea. Unfortunately, the metal-metal interaction in polynuclear s* complexes has been characterized almost only by structural data. However, since s* metal ions are generally luminescent [3], absorption and emission spectroscopy should be well suited for studying the metal-metal interaction in these clusters. This expectation is based on a certain analogy between the electronic structure of s* and d”’ metal clusters. In the ground state of d” clusters a weak metal-metal bonding is accomplished by ds (or dp) orbital mixing [4]. Many polynuclear d” complexes are luminescent [S]. In the ds excited state some tetranuclear clusters seem to undergo a considerable contraction by the promotion of an electron from an antibonding d to a bonding s orbital [6]. As consequence the emission undergoes a large red shift with respect to the absorption. A similar effect can be anticipated for s* clusters. This assumption is supported by an observation that dimer formation of Bi3+ ions in solid matrices is accompanied by a change of the luminescence of the s* ion [7]. For the present study we selected the compound [T10CH3]4 since it has a simple composition and structure [8]. In addition, the Tl+ ion is a strong emitter in solution under ambient conditions [3].